1. Field of the Invention
The present invention relates to a driving circuit of a high-power semiconductor laser.
2. Description of the Prior Art
Conventionally, as a driving circuit of a semiconductor laser, that as shown in FIG. 3 is employed. The driving circuit of a semiconductor laser includes a monitor type photodiode 32, a bias current circuit 34, a high-frequency superimposing circuit 35 and a coupling capacitor 33. When a semiconductor laser 31 is employed with low output for reading data in an optical magnetic disc, for example, a driving bias current Ib supplied by the bias current circuit 34 is modulated by a high frequency, and the high-frequency-modulated current is used to drive the semiconductor laser 31. At this time, the photodiode 32 receives a light emitted by the semiconductor laser 31 and outputs an optical output detecting signal expressing an optical output value of the semiconductor laser 31 to the bias current circuit 34. Then, the bias current circuit 34 regulates the bias current Ib so that the mean value of the optical output values of the semiconductor laser 31 keeps constant. An example of an operating state of the semiconductor laser 31 is shown in FIG. 4. In FIG. 4, F is an optical output characteristic of the semiconductor laser 31, while Q.sub.0 is a high-frequency-modulated waveform of the high-frequency superimposing circuit 35. Moreover, Ith denotes a threshold current of the semiconductor laser 31; when a current flowing into the semiconductor laser 31 is the threshold current Ith or under, an optical output P of the semiconductor laser 31 becomes zero, and the oscillation of the semiconductor laser 31 ceases. The optical output characteristic F of the semiconductor laser 31 is defined by an optical output characteristic value, such as a differential efficiency .eta. which is a variation in the optical output P related to a variation in the bias current Ib in an induced emission region, the above-mentioned threshold current Ith and the like. The optical output waveform P.sub.0 of the semiconductor laser 31 is, as shown in FIG. 4, adapted to intermittently have periods of time for which it becomes zero. For this reason, the semiconductor laser 31 intermittently repeats a halt and resumption of laser oscillation, and it comes to easily cause a multimode oscillation. The semiconductor laser 31 decreases a noise, such as a mode hopping noise, a noise resulting from a return light and the like, to the optical output of the semiconductor laser 31.
In general, however, semiconductor lasers have their respective significantly different optical output characteristic values, such as the threshold current Ith, the differential efficiency .eta. in the induced emission region and the like, which are determined by the optical output characteristics of the semiconductor lasers. Thus, in the conventional driving circuit of the semiconductor laser, because of an unevenness of the optical output characteristic values of the semiconductor laser 31, the optical output waveform of the semiconductor laser 31 does not identical with the optical output waveform P.sub.0 having periods of time for which the optical output value becomes zero as shown in FIG. 4, and it is possible that there is no period of time for which the optical output value becomes zero. In this case, since the semiconductor laser 31 does not intermittently repeat a halt and resumption of laser oscillation, the multimode oscillation by which the noise, such as the mode hopping noise and the like, to the optical outputs can be reduced is not easily caused.
The mode hopping noise herein means a noise caused when a vertical mode hops due to a slight change in temperature or the like in a laser without hysteresis.
U.S. Pat. No. 4,480,325 discloses an optical pickup provided with a semiconductor laser device which uses D. C. current and a high frequency current superposed thereon to project laser beam onto an optical disc.